Prevent monster generation in the sokoban trap hallway

Makes Sokoban far less tedious when you don't have to worry about
monsters randomly popping up in the trap hallway while you're pushing
the boulder.

Adds a new exclusion zone for monster generation, and the goodpos
routine avoids the zones when GP_AVOID_MONPOS is used.

dat/soko3-2.lua

@@ -47,6 +47,8 @@ des.object("boulder",07,10)
 des.object("boulder",10,10)
 des.object("boulder",03,11)
 
+-- prevent monster generation over the (filled) holes
+des.exclusion({ type = "monster-generation", region = { 12,10, 24,10 } });
 -- Traps
 des.trap("hole",12,10)
 des.trap("hole",13,10)